How do possums get in
They go around every night sticking their nose in and poking around to see if there is a weak point that may allow them access.
They will then chew and scratch and see if they can get in.
With a bit of effort they can often force their way in.
Maintenance is the key
The way to keep on top of them is to do regular maintenance checks to ensure you have no loose or broken tiles, rotted or decaying timber (particularly the fascia and barge boards)
Limit access to the roof by trimming back trees close to the house.